1. Field of the Invention
The present invention relates to a ceramic multilayer substrate that is formed by stacking and firing a plurality of insulating layers made of ceramic materials.
2. Description of the Related Art
Ceramic multilayer substrates are widely used as substrates on which circuits of various electronic devices, e.g., cellular phones, are formed. A manufacturing method for the ceramic multilayer substrate includes, for example, the steps of forming a via hole in a ceramic green sheet, filling a conductive material in the via hole to form a via conductor that serves as an interlayer connection conductor, and then forming wiring electrodes on a principal surface of the ceramic green sheet. The ceramic multilayer substrate is obtained by stacking and firing the plural ceramic green sheets each of which has been formed in a similar manner.
It is known that the above-mentioned type of ceramic green sheet is contracted when fired. Furthermore, individual ceramic green sheets undergo variations in amounts of contraction thereof. In particular, if variations in amounts of contraction occur in the direction of principal surfaces (i.e., in the plane direction) of the ceramic green sheets, positions of the wiring electrodes and the via conductors formed in the individual ceramic green sheets are deviated, thus causing a possibility that proper connection between the wiring electrodes and the via conductors, which are to be interconnected, may not be obtained between the adjacent ceramic green sheets. In recent years, the above-mentioned problem has become more significant because further reduction in width of the wiring electrodes and size of the via conductors formed on the ceramic multilayer substrate has been demanded with downsizing of electronic devices.
In view of the situations described above, a ceramic multilayer substrate has been proposed which can suppress contraction of the ceramic multilayer substrate caused in the plane direction during firing (see Japanese Unexamined Patent Application Publication No. 2008-186909 (paragraphs 0015 to 0018, FIG. 1). In a proposed ceramic multilayer substrate 100, as illustrated in FIG. 5, a first insulating layer 101a and a second insulating layer 101b are alternately stacked, and various wiring electrodes 102 and via conductors 103 are formed inside or on surfaces of the insulating layers. In this case, the first insulating layer 101a and the second insulating layer 101b are made of glass ceramics that have contraction starting temperatures and contraction end temperatures different from each other in firing of the ceramic multilayer substrate 100. Respective materials of the first insulating layer 101a and the second insulating layer 101b are selected such that contraction with firing of the second insulating layer 101b starts after the end of contraction with firing of the first insulating layer 101a. 
By selecting the materials as mentioned above, because the contraction with firing of the adjacent second insulating layer 101b is not yet started at the time of the start of the contraction with firing of the first insulating layer 101a, the contraction of the first insulating layer 101a in the direction of the principal surface (i.e., in the plane direction) is suppressed. Moreover, because the contraction with firing of the adjacent first insulating layer 101a is already ended at the time of the start of the contraction with firing of the second insulating layer 101b, the contraction of the second insulating layer 101b in the plane direction is suppressed. As a result, the contraction of the ceramic multilayer substrate in the plane direction is suppressed, and the positions where the wiring electrodes 102 and the via conductors 103 are formed on and in the insulating layers 101a and 101b are prevented from being deviated when the ceramic multilayer substrate 100 is fired.
However, the above-described ceramic multilayer substrate 100 has a property that the contraction in the plane direction is suppressed, while an amount of contraction in a direction (stacking direction) perpendicular to the plane direction is increased. Furthermore, the via conductors 103 formed inside the ceramic multilayer substrate 100 are contracted during the firing without substantially being affected by the insulating layers 101a and 101b, and the amount of contraction of each via conductor 103 in the stacking direction is smaller than the amount of contraction of each of the insulating layers 101a and 101b in the stacking direction. Therefore, when another via conductor 103 is arranged just under some one via conductor 103 formed in an uppermost surface layer, for example, there occurs a phenomenon that, during the firing of the ceramic multilayer substrate 100, the via conductor 103 in the uppermost surface layer is pushed up by the other via conductor 103 and is protruded from the surface of the ceramic multilayer substrate 100. Such a protrusion of the via conductor 103 is undesired because the protrusion may deteriorate mountability of a component that is mounted to the surface of the ceramic multilayer substrate 100.
In the ceramic multilayer substrate 100, to cope with the above-mentioned problem, the via conductor 103 in the uppermost surface layer is formed to have a smaller sectional area on the uppermost surface side than a sectional area on the inner layer side. As a result, the protrusion of the via conductor 103 in the uppermost surface layer is prevented for the reason that, during the firing, the insulating layers 101a and 101b covering lateral peripheral surfaces of the via conductor 103 in the uppermost surface layer function as resistances against pushing by the via conductor 103, which is positioned just under the above-mentioned via conductor 103.
On the other hand, when terminals of a surface-mounted component mounted to the surface of the ceramic multilayer substrate 100 are connected to exposed end surfaces of the via conductors 103 in the uppermost surface layer, strength in connection of the component is increased in some cases by separately forming land electrodes for mounting on the relevant via conductors 103, and by connecting the land electrodes to the component terminals. However, because the land electrodes are formed in larger areas than areas of the end surfaces of the via conductors 103 in the uppermost surface layer, which are exposed at the surface of the ceramic multilayer substrate 100, a difficulty arises in adaptation to mounting of a component, such as an IC, having terminals arranged at a high density.
To be adapted for mounting of the high-density component, a technique for directly connecting the via conductors 103 to the component terminals without providing the land electrodes has been proposed in recent years. However, when the proposed technique is applied to the above-described ceramic multilayer substrate 100, sufficient strength in connection to the surface-mounted component cannot be obtained in some cases because the via conductor 103 in the uppermost surface layer is formed to have a sectional area that gradually reduces toward the uppermost surface side.
One conceivable solution to the above-mentioned problem is to form the via conductor 103, which is positioned in the uppermost layer of the ceramic multilayer substrate 100 and is directly connected to the terminal of the surface-mounted component, in such a shape that a sectional shape of the via conductor 103 gradually increases toward the uppermost surface side, thereby increasing an area where the via conductor 103 is connected to the surface-mounted component. In the above-described ceramic multilayer substrate 100, however, the contraction of the insulating layers 101a and 101b is suppressed during the firing, whereas the contraction of the via conductor 103 is not suppressed. Accordingly, a difference in amount of contraction during the firing is caused between each of the insulating layers 101a and 101b and the via conductor 103, and a gap is generated between the via conductor 103 and each of the insulating layers 101a and 101b. 
If moisture enters the ceramic multilayer substrate 100 through the above-mentioned gap, there is a possibility that insulation resistances of the insulating layers 101a and 101b may be reduced due to electrochemical migration occurred inside the ceramic multilayer substrate 100, or that the moisture having entered the inside may be expanded with, e.g., heat applied when the surface-mounted component is mounted, thus causing damage to the insulating layers 101a and 101b due to an expansion force of the moisture. Furthermore, if the expansion force acts on a solder for mounting of the component when the solder is in a molten state, there is a possibility of the so-called solder flash that the solder is scattered to the surroundings and the adjacent via conductors are short-circuited.